1. Field of the Invention
The present invention relates to a millimeter-wave transmitter-receiver using a non-radiative dielectric waveguide (i.e., NRD guide) employed in a millimeter-wave radar module, a millimeter-wave wireless telegram or the like and, more particularly, to a suppression of fluctuations of an output signal coming from a mixer due to the transient characteristics of a pulse modulator while such a switching controller of the millimeter-wave transmitter-receiver being closed (ON) as can shield the pulse-modulated transmission millimeter wave signal, which might otherwise be outputted to a reception system by an internal reflection or the like. The invention further relates to a transmitting/receiving antenna, in which an antenna (including a primary radiator) is connected to one of inputting/outputting transmission lines of a circulator, for preventing a portion of a millimeter-wave signal of a transmission system from leaking directly to the reception system.
Furthermore, the invention relates to an isolator for a high-frequency signal, which is used in a millimeter-wave integrated circuit, a millimeter-wave radar module or the like and, more particularly, to a circulator type isolator, in which a non-reflective terminator is connected to one of the inputting/outputting transmission lines of the circulator and which can improve the isolation characteristics, and to a high-frequency oscillator and high-frequency transmitter-receiver using the isolator.
Moreover, the invention relates to a high-frequency oscillator packaged in a non-radiative dielectric waveguide type millimeter-wave integrated circuit, a millimeter-wave radar module or the like for generating a millimeter-wave signal with a high-frequency diode, and a non-radiative dielectric waveguide type high-frequency transmitter-receiver using the oscillator.
Moreover, the invention relates to a high-frequency transmitter-receiver to be employed in a millimeter-wave radar module, a millimeter-wave wireless telegram or the like acting as a radar device mounted on a vehicle, a small-sized boat or the like and, more particularly, to a high-frequency transmitter-receiver for preventing a portion of the transmission high-frequency signal from being transmitted as an unnecessary signal when a modulator is OFF, and exerting adverse effects on the transmission/reception of the high-frequency signal.
Moreover, the invention relates a radar apparatus provided with the high-frequency transmitter-receiver, and a radar-apparatus mounted vehicle and radar-apparatus mounted small-sized boat provided with that radar apparatus.
2. Description of the Related Art
In the related art, for example, a pulse modulation system, as disclosed in Japanese Unexamined Patent Publication JP-A 2000-258525, has been proposed as a system which is expected as an application to a millimeter-wave radar module, a millimeter-wave wireless telegram or the like.
In the pulse modulation system, however, a portion of the pulse-modulated transmission millimeter-wave signal is outputted as an unnecessary signal to the reception system by the reflection or the like in the transmitter-receiver. This output raises a problem that the reception performance is adversely affected.
We have already proposed a solution for that problem (as referred to Japanese Unexamined Patent Publication JP-A 2003-198421). An example of the proposal is shown in a block circuit diagram of FIG. 28 and in a top plan view of FIG. 6. A fundamental configuration of the NRD guide, as used in the configuration example, is shown in a partially broken perspective view of FIG. 4. In this configuration, a dielectric guide 3 is sandwiched between parallel flat plate conductors 1 and 2 arranged in parallel at a spacing a.
The millimeter-wave transmitter-receiver shown in FIG. 6 presents an example in which a transmitting antenna and a receiving antenna are integrated. The millimeter-wave transmitter-receiver includes a pair of parallel flat plate conductors 51, a first dielectric guide 53, a millimeter-wave signal oscillator 52, a pulse modulator (not-shown), a second dielectric guide 58, a circulator 54, a third dielectric guide 55, a fourth dielectric guide 57, and a mixer 59. The pair of parallel flat plate conductors 51 is arranged in parallel at a spacing of one half or less of the wavelength of the millimeter-wave signal. The first dielectric guide 53 is disposed between the parallel flat plate conductors 51. The millimeter-wave signal oscillator 52 is disposed between the parallel flat plate conductors 51, is attached to a first dielectric guide 53, frequency-modulates a high-frequency signal outputted from a high-frequency diode and thereby the modulated signal as a millimeter-wave signal is propagaed via the first dielectric guide 53. The pulse modulator is disposed between the parallel flat plate conductors 51, is disposed midway of the first dielectric guide 53, pulsates the millimeter-wave signal and outputs the pulsated signal as a transmission millimeter-wave signal from the first dielectric guide 53. The second dielectric guide 58 is disposed between the parallel flat plate conductors 51, is disposed so that its one end side is close to the first dielectric guide 53 or joined at its one end to the first dielectric guide 53 to obtain electromagnetic coupling, and propagates a portion of the millimeter-wave signal.
The circulator 54 is disposed between the parallel flat plate conductors 51. Further, the circulator 54 has ferrite plates arranged in parallel with the parallel flat plate conductors 51, a first connecting portion 54a, a second connecting portion 54b and a third connecting portion 54c. The first to third connecting portions 54a to 54c are arranged at a predetermined spacing at the peripheral edge portion of the ferrite plates and individually act as the input/output terminals of the millimeter-wave signal. The circulator 54 outputs the millimeter-wave signal inputted from one connecting portion, from another connecting portion adjoining clockwise or counter-clockwise in the plane of the ferrite plates. In the circulator 54, the first connecting portion 54a is connected to the millimeter-wave signal output terminal of the first dielectric guide 53. The third dielectric guide 55 is disposed between the parallel flat plate conductors 51, is connected to the second connecting portion 54b of the circulator 54, has an antenna 56 at its leading end portion, and propagates the millimeter-wave signal. The fourth dielectric guide 57 is disposed between the parallel flat plate conductors 51, is connected to the third connecting portion 54c of the circulator 54, and propagates the received wave, which is received by the antenna 56 and outputted via the third dielectric guide 55 and the second connecting portion 54b from the third connecting portion 54c. 
The mixer 59 is disposed between the parallel flat plate conductors 51, is configured so that either a midway of the second dielectric guide 58 and a midway of the fourth dielectric guide 57 are close to or joined to each other to obtain electromagnetic coupling, and mixes the millimeter-wave signal portion having propagated via the second dielectric guide 58 and the received wave having propagated via the fourth dielectric guide 57 thereby to generate an intermediate-frequency signal. In this example, moreover, the mixer 59 is provided at its output end with a (not-shown) switching controller for closing the output terminal when the pulse-modulated transmission millimeter-wave signal is outputted from the pulse modulator. Thus, the unnecessary signal can be prevented from being outputted to the reception system at a downstream stage of the mixer 59, substantially simultaneously as a pulsated signal for starting the pulsating operation of the pulse modulator is inputted to the pulse modulator.
Next, FIG. 28 is a block circuit diagram showing the configuration of the individual units at the time when the millimeter-wave transmitter-receiver shown in FIG. 6 is used as a mill-wave radar.
In FIG. 28, a voltage-controlled oscillator (abbreviation: VCO) 411 is provided with a Gunn diode and a varactor diode. This VCO 411 acts when a signal is inputted to its IN-2 terminal for inputting the modulated signal. A pulse modulator 412 performs pulse modulations when the pulsated signal inputted to an IN-1 terminal is inputted to the pulse modulator 412. In FIG. 6, this pulse modulator 12 is a switch disposed midway of the first dielectric guide 53 and is configured, as shown in a perspective view of FIG. 32.
The pulse modulator shown in FIG. 32 is configured into a switch, in which a choke type bias feed line 90 is formed on one principle face of a substrate 88 and in which a beam lead type PIN diode or a Schottky barrier diode 80 is soldered between connecting electrodes 81 and 81 formed midway of the bias feed line 90. This switch is employed as the pulse modulator 412 by disposing the PIN diode or the Schottky barrier diode 80 between the end faces midway of the first dielectric guide 53 in a manner to have a transverse bias applying voltage direction.
Referring to FIG. 28, the millimeter-wave transmitter-receiver comprises a circulator 413 for transmitting the millimeter-wave signal to an antenna 414 at a transmitting time and for transmitting the received wave to a mixer 415 described later at a receiving time, an antenna 414 for transmitting/receiving a millimeter-wave signal, which is exemplified by a horn antenna or the like, which is connected to the circulator 413 through a metal waveguide or a dielectric waveguide prepared by filling a metal waveguide with a dielectric. The millimeter-wave transmitter-receiver further comprises a mixer 415 for mixing the millimeter-wave signal outputted from the VCO 411 and the received signal received at the antenna 414 thereby to output an intermediate-frequency signal for detecting the range or the like to a target, a switch 416 for shielding or passing the intermediate-frequency signal outputted from the mixer 415, and a controller 419 for controlling the switching (or ON/OFF) timing of the switch 416. The switch 416 and controller 419 configure the switching controller.
The controller 419 receives the pulsated signal of the IN-1 terminal in association with the pulse modulator 412 and controls the ON/OFF timing of the switch 416 so that the transmission millimeter-wave signal pulse-modulated by the pulse modulator 412 may be shielded by the switch 416, before the pulse-modulated millimeter-wave signal is reflected by the connecting portion between the NRD guide and the dielectric waveguide or leaks from the circulator 413 and is outputted as an unnecessary signal through the mixer 415 to an amplifier 418.
Referring to FIG. 28, the millimeter-wave transmitter-receiver further comprises a capacitor 417 for AC-coupling the switch 416 and the amplifier 418.
With the configuration thus far described, the pulse-modulated transmission millimeter-wave signal can be so shielded as may not be mixed into the mixer 415 to leak into the reception system at a down stream stage. Thus, it is possible to enhance the detection precision of the millimeter-wave radar system.
In one known transmitting/receiving antenna to be assembled for use into such millimeter-wave transmitter-receiver, on the other hand, a radiator is connected to one inputting/outputting transmission line of the circulator. This example of the transmitting/receiving antenna is disclosed in not only the aforementioned Japanese Unexamined Patent Publication JP-A 2000-258525 but also Japanese Unexamined Patent Publication JP-A 7-77576.
The transmitting/receiving antenna of the related art, as disclosed in Japanese Unexamined Patent Publication JP-A 2000-258525 or Japanese Unexamined Patent Publication JP-A 7-77576, is configured, as shown in a top plan view of FIG. 29, for example, such that the antenna 428 is connected to the other end of the third transmission line 426, which is connected at its one end with one connecting portion of such a circulator that the first, second and third transmission lines 424, 425 and 426 for transmitting the millimeter-wave signal are radially connected at the peripheral edge portion of the magnetic element 427 thereby to output the millimeter-wave signal inputted from one connecting portion, from another adjacent connecting portion.
In the transmitting/receiving antenna of the related art, the millimeter-wave signal outputted from the transmission system is inputted to the first transmission line 424 and outputted from the first transmission line 424 to the third transmission line 426 so that the mill-wave signal inputted to the third transmission line 426 is transmitted from the antenna 428 connected to the third transmission line 426. At the same time, the mill-wave signal received by the antenna 428 is inputted to the third transmission line 426 and is outputted from the third transmission line 426 to the second transmission line 425 so that the mill-wave signal is outputted from the second transmission line 425 to the reception system. Thus, the transmitting/receiving antenna shares the single antenna 428 and can transmit/receive the mill-wave signal.
In the configuration proposed by Japanese Unexamined Patent Publication JP-A 2003-198421, however, we have made keen investigations for enhancing the performance of the millimeter-wave transmitter-receiver more and have found the following problems desired to be improved.
The problem desired to be improved is to consider such a fluctuation of the output level of the transmission mill-wave signal from the pulse modulator 412 as accompanies the transient response characteristics of the pulse modulator 412.
Generally, the pulse modulator 412 using the high-frequency diode has the characteristics intrinsic to those of the high-frequency diode such as a zero-bias capacitor. Even if an ideal pulse signal is inputted for the driving purpose, the modulating bias current is accompanied in most cases by the transient fluctuation such as the ringing noise. As a result, the mill-wave signal output from the pulse modulator 412 is distorted from its original waveform thereby to raise a problem that the signal output is accompanied by the level fluctuation distorted from the design concept.
The millimeter-wave transmitter-receiver, as proposed in Japanese Unexamined Patent Publication JP-A 2003-198421, is provided with the switch 416 for preventing the transmission mill-wave signal outputted from the pulse modulator 412 from migrating directly into the mixer 415 by the reflection or the like and from being outputted to the reception system. It is, however, necessary to shield the unnecessary signal which is generated in the reception system, i.e., in the mixer 415 by the aforementioned level fluctuation. Before the aforementioned level fluctuation is converged and stabilized to a steady state, therefore, the switch 416 cannot be turned ON thereby to raise a problem that the transmission/reception cannot be performed for a constant time period just after the pulse signal was sent out.
The problem desired to be improved is to suppress the level fluctuation of the output of the mixer 415 due to the transient fluctuation of the output level of the mill-wave signal from the pulse modulator 412.
For one countermeasure for solving this problem, an effective method is to combine the pulse modulators 412 in multiple stages thereby to improve the ON/OFF ratio characteristics of the pulse modulators 412 so that the especially serious problem of the level fluctuation at the OFF state time of the pulse modulators 412 may be suppressed. For this effective method, the configuration is complicated to raise problems that the increase in the number of steps of assembling the millimeter-wave transmitter-receiver and the enlargement of the sizes of the millimeter-wave transmitter-receiver itself and the millimeter-wave radar apparatus using the millimeter-wave transmitter-receiver are invited.
Moreover, the transmitting/receiving antenna of the related art, as disclosed in Japanese Unexamined Patent Publication JP-A 2000-258525 or Japanese Unexamined Patent Publication JP-A 6-188633, has a problem that the portion of the mill-wave signal inputted from the transmission system leaks from the first dielectric guide 424 to the second dielectric guide 425 and is mixed with the millimeter-wave signal to be received, thereby to deteriorate the reception characteristics of the reception system.
Further, as the isolator for the high-frequency signal assembled for use in the millimeter-wave integrated circuit or the millimeter-wave radar module, there has been known in the related art a circulator type isolator, in which a non-reflective terminator is connected to one of the inputting/outputting transmission lines of the circulator. An example of this circulator type isolator of the related art is disclosed in Japanese Unexamined Patent Publication JP-A 7-235808, for example.
The isolator disclosed in Japanese Unexamined Patent Publication JP-A 7-235808 is configured, as shown in the schematic top plan view of FIG. 30, for example, such that the non-reflective terminator 438 is connected to the other end of the third transmission line 436, which is connected at its one end with one connecting portion of such a circulator that the first, second and third transmission lines 434, 435 and 436 for transmitting the high-frequency signal are radially connected at the peripheral edge portion of the magnetic element 437 thereby to output the millimeter-wave signal inputted from one connecting portion, from another adjacent connecting portion, and such that the line length of the third transmission line 436 is set to (2n+1)/4·λg (n: an integer, and λg: the wavelength of the high-frequency signal in the third transmission line 436).
According to this isolator of the related art, when the high-frequency signal inputted from the first transmission line 434 is outputted from the second transmission line 435 whereas that portion of the high-frequency signal, which is reflected on the output terminal end of the second transmission line 435, is inputted to the third transmission line 436 and is terminated at the non-reflective terminator 438 so that the high-frequency signal portion reflected on the output terminal side of the second transmission line 435 may not leak to the input terminal of the first transmission line 434. At the time the high-frequency signal, which is not fully terminated at the non-reflective terminator 438 but reflected to leak from the third transmission line 436 to the first transmission line 434, and the high-frequency signal portion, which is reflected on the output terminal of the second transmission line 435 to leak from the second transmission line 435 to the first transmission line 434, are made to interfere so that the high-frequency signal reflected on the output terminal of the second transmission line 435 to leak to the input terminal of the first transmission line 434 can be more attenuated. Therefore, it is possible to make the isolation characteristics satisfactory.
In the related art, on the other hand, the metal waveguide is often used for transmitting the high-frequency signal of a micro-wave or millimeter-wave. By the demand of recent years for reducing the size of the high-frequency module, however, there has been developed the high-frequency module which uses the dielectric guide as the waveguide of the high-frequency signal. Of these, the non-radiative dielectric waveguide (as will also called the “NRD guide”) having little transmission loss of the high-frequency signal is being noted.
The fundamental configuration of the NRD guide is shown in a partially broken perspective view in FIG. 411. As shown in FIG. 411, the NRD guide is configured by arranging the dielectric guide 3 having a rectangular sectional shape such as a rectangle between the parallel flat plate conductors 1 and 2 arranged in parallel at a predetermined spacing a. In case this spacing a is defined by a ≦λ/2 with respect to the wavelength λ of the high-frequency signal, the high-frequency signal can be efficiently propagated in the dielectric guide 3 by eliminating the intrusion of the noise from the outside into the dielectric guide 3 and by eliminating the radiation of the high-frequency signal to the outside. The wavelength λ of the high-frequency signal is the value in the air (or the free space) in the operating frequency.
An example of the high-frequency oscillator of the related art to be incorporated into the NRD guide is shown in perspective views in FIG. 31 and FIG. 32. FIG. 31 is a perspective view showing the example of the high-frequency oscillator of the related art, and FIG. 32 is a perspective view of a wiring substrate, which is provided with a variable capacity diode (i.e., a varactor diode) for the high-frequency oscillator. In FIG. 31 and FIG. 32, the parallel flat plate conductors are not shown. This high-frequency oscillator oscillates the frequency-modulated high-frequency signal by using the Gunn diode and the varactor diode in combination, and a high-frequency transmitter-receiver, a millimeter-wave radar module or the like using such-high-frequency oscillator has been developed. FIG. 33 is a top plan view showing an example of the millimeter-wave radar module, which is configured by incorporating the high-frequency oscillator of the related art as a millimeter-wave signal oscillator 502.
The high-frequency oscillator shown in FIG. 31 is configured to include a voltage-controlled oscillator V and a circulator E. At first, the voltage-controlled oscillator V or the component of the high-frequency oscillator shown in FIG. 31 has the configuration to be described in the following. In FIG. 31 and FIG. 32: reference numeral 82 denotes a metal member of a generally box-shaped metal block or the like for mounting a Gunn diode 83; the reference numeral 83 denotes the Gunn diode or a kind of a high-frequency diode for oscillating millimeter-waves; reference numeral 84 denotes a wiring substrate disposed on one side face of the metal member 82 and having a choke type bias feed line 84a formed to function as a low-pass filter for feeding a bias voltage to the Gunn diode 83 and for preventing the leakage of the high-frequency signal; reference numeral 85 denotes a band-shaped conductor such as a metal foil ribbon for connecting the choke type bias feed line 84a and the upper conductor of the Gunn diode 83; reference numeral 86 denotes a metal strip resonator having a resonating metal strip line 86a disposed on the dielectric substrate; and reference numeral 87 denotes a dielectric guide for guiding the high-frequency signal resonated by the metal strip resonator, to the outside of the millimeter-wave signal oscillator.
Midway of the dielectric guide 87, moreover, there is disposed the wiring substrate 88, in which the wiring substrate 88 having the varactor diode 80 or a frequency modulating diode, i.e., a kind of variable capacity diode packaged therein. The bias voltage applying direction of the varactor diode 80 is made perpendicular to the propagation direction of the high-frequency signal in the dielectric guide 87 and parallel (i.e., in the static direction) to the principal face of the parallel flat plate conductors. Moreover, the bias voltage applying direction of the varactor diode 80 is aligned with the static direction of the high-frequency signal in an LSM01 mode to propagate in the dielectric guide 87. By coupling the high-frequency signal and the varactor diode 80 electromagnetically to control the bias voltage thereby to vary the electrostatic capacity of the varactor diode 80, therefore, the frequency of the high-frequency signal can be controlled. Moreover, reference numeral 89 denotes a dielectric plate having a high specific dielectric constant for matching the impedance between the varactor diode 80 and the dielectric guide 87.
As shown in FIG. 32, the second choke type bias feed line 90 is formed on one principal face of the wiring substrate 88, and the varactor diode 80 is arranged midway of the second choke type bias feed line 90. The connecting electrode 81 is formed at the connecting portion of the second choke type bias feed line 90 with the varactor diode 80.
The high-frequency signal oscillated from the Gunn diode 83 is derived through the metal strip resonator 86 to the dielectric guide 87. Next, a portion of the high-frequency signal is reflected on the varactor diode 80 and returned to the Gunn diode 83. This reflected signal changes according to the variation in the static capacity of the varactor diode 80 so that its oscillatory frequency varies.
Next, the circulator E, i.e., the component of the high-frequency modulator shown in FIG. 31 includes a first connecting portion 92a, a second connecting portion 92b and a third connecting portion 92c arranged at a predetermined spacing at the peripheral edge portions of two ferrite plates 91a and 91b arranged in parallel with the parallel flat plate conductor, for individually acting as input/output terminals of the mill-wave signal. In the circulator E, one end of the dielectric guide 87, one end of a dielectric guide 93 and one end of a dielectric guide 94 are connected to the first connecting portion 92a, second connecting portion 92b and the connecting portion 92c, respectively, such that the mill-wave signal inputted from the other end of the dielectric guide 87 is outputted from an output terminal 93a or the other end of the dielectric guide 93 adjoining counterclockwise in the planes of the ferrite plates 91a and 91b. In the circulator E, a portion of the output is reflected back and inputted from the output terminal 93a, and it is inputted to one end of the dielectric guide 94 adjoining counterclockwise in the planes of the ferrite plates 91a and 91b and outputted from the other end.
The high-frequency modulator shown in FIG. 31 is configured such that the voltage-controlled oscillator V and the circulator E are connected via the dielectric guide 87, and such that a non-reflective terminator 95 is connected to the other end of the dielectric guide 94. The mill-wave signal generated by the voltage-controlled oscillator V is transmitted from the first connecting portion 92a of the circulator E to the second connecting portion 92b and is extracted as the millimeter-wave oscillation output from the output terminal 93a. The circulator E and the non-reflective terminator 95 act as the isolator to isolate the voltage-controlled oscillator V and the output terminal 93a so that the millimeter-wave oscillation output of the voltage-controlled oscillator V may not return to the voltage-controlled oscillator V. Thus, the voltage-controlled oscillator V oscillates stably. This technique on the high-frequency oscillator is disclosed in Japanese Unexamined Patent Publication JP-A 6-188633, JP-A 6-177650, JP-A 6-177649 and JP-A 6-97735, for example.
On the other hand, the millimeter-wave radar module shown in FIG. 33 is of the FMCW (Frequency Modulation Continuous Waves) type having the following operation principle. An input signal having a voltage amplitude changing in a triangular wave, a sinusoidal wave or the like with time is inputted to a modulated signal inputting terminal of the mill-wave signal oscillator 502 made of the high-frequency oscillator, as shown in FIG. 31, the output signal of which is frequency-modulated to deviate the output frequency of the mill-wave signal oscillator 502 into the triangular wave, the sinusoidal wave or the like. In case an output signal (or a transmission wave) is radiated from a transmitting/receiving antenna 506, the reflected wave (or the received wave) returns from a target, if any in front of the transmitting/receiving antenna 506, with a time difference for reciprocations of the propagation velocity of the electric waves. At this time, the intermediate-frequency signal corresponding the frequency difference between the transmitted wave and the received wave is outputted to the intermediate-frequency output terminal on the output side of a mixer 510.
By analyzing the frequency component such as the output frequency of the output of that intermediate-frequency output terminal, the range to the target can be derived from a relational Formula: Fif=4R·fm·Δf/c (Fif: IF (Intermediate Frequency) output frequency; R: a range; fm: a modulated frequency; Δf: a frequency deviation width; and c: the velocity of light).
The technique for the millimeter-wave radar module using that high-frequency oscillator is disclosed in Japanese Unexamined Patent Publication JP-A 6-174824, JP-A 10-22864 and JP-A 10-224257, for example.
On the other hand, the examples of the radar apparatus of the related art and the radar-apparatus mounted vehicle having the radar apparatus mounted thereon are disclosed in Japanese Unexamined Patent Publication JP-A 2003-35768, for example.
In the isolator of the related art disclosed in Japanese Unexamined Patent Publication JP-A 7-235808, however, the advancement in the phase of the high-frequency signal changes in fact at the time when the high-frequency signal is reflected mainly on the non-reflective terminator 438. With the aforementioned setting of the line length of the third transmission line 436 to improve the isolation characteristics under the premise of no phase change, the two high-frequency signals to leak to the first transmission line 14 deviate from the opposite phases so that they are synthesized. This synthesization raises a problem that it is impossible to sufficiently attenuate the high-frequency signal which might otherwise be reflected on the output terminal of the second transmission line 435 to leak to the input terminal of the first transmission line 434.
On the other hand, the high-frequency oscillator of the related art shown in FIG. 31 has a narrow frequency band width for a high isolation, as the isolation characteristics of one stage of the circulator or the component of the high-frequency modulator are shown in the graph of FIG. 16. This raises a problem that the oscillatory frequency band is limited (e.g., 1 GHZ or less in the example of FIG. 16) is restricted fro the stable oscillation of the high-frequency oscillator within the range where the isolation can be at a predetermined or higher value of 30 dB or more.
In case the high-frequency oscillator of the related art is incorporated for use into the millimeter-wave radar module or the like, this millimeter-wave radar module is mounted in such an engine room or the like of an automobile as has serious temperature variations. The oscillatory frequency of the high-frequency oscillator depends on the temperature. This dependency raises a problem that the high-frequency oscillator outputs millimeter-wave oscillations with such a frequency thereby to deteriorate the radar detecting performance that the isolation of the circulator cannot be made at the environmental temperature.
The mill-wave oscillation output may be further pulse-modulated in the millimeter-wave radar module of the related art to perform the millimeter-wave transmission/reception of less noises. In this case, the pulsating mill-wave signal returns from the pulse modulator to the high-frequency oscillator of the related art so that a more serious factor for the oscillation instability is added to the voltage-controlled oscillator. This addition raises a problem that the isolation of the circulator becomes short.
Against these problems, on the other hand, there is conceived solution means for configuring a similar high-frequency oscillator by using the circulator having the two-stage configuration, as has been proposed by us in Japanese Unexamined Patent Publication JP-A 2003-218609. In this technique, however, a more improvement has been desired for widening the frequency band width, in which an isolation at a predetermined or higher value is retained.
Further, in the related art, for example, a pulse modulation system, as disclosed in Japanese Unexamined Patent Publication JP-A 2000-258525, has been proposed as the system which is expected as an application to a millimeter-wave radar module, a millimeter-wave wireless telegram or the like.
The high-frequency transmitter-receiver of this pulse modulation type of the related art is configured, as shown in a schematic block circuit diagram of FIG. 34, for example, to include a high-frequency oscillator 61, a branching device 62, a pulse modulator 63, a circulator 64, a transmitting/receiving antenna 65, and a mixer 66. The high-frequency oscillator 61 is generated a high-frequency signal. The branching device 62 is connected to the output terminal of the high-frequency oscillator 61, and branches the high-frequency signal and outputs the branched signals to one output terminal 62b and the other output terminal 62c. The pulse modulator 63 is connected to the one output terminal 62b of the branching device 62 for pulse-modulating a portion of the high-frequency signal to output the modulated signal as a transmission high-frequency signal. The circulator 64 has first, second and third terminals 64a, 64b and 64c. In the circulator 64, the first terminal 64a is connected to the output terminal 63a of the pulse modulator 63, and the high-frequency signal inputted from the first terminal 64a output to the second terminal 64b and the high-frequency signal inputted from the second terminal 64b output to the third terminal 64c. The transmitting/receiving antenna 65 is connected to the second terminal 64b of the circulator 64. The mixer 66 is connected between the other terminal 62c of the branching device 62 and the third terminal 64c of the circulator 64. The mixer 66 output an intermediate-frequency signal with mixing the high-frequency signal as a local signal LO outputted to the other output terminal 62c of the branching device 62 and a high-frequency signal RF received by the transmitting/receiving antenna 65.
Other examples of the high-frequency transmitter-receiver of the related art adopting that pulse modulation type are disclosed in Japanese Unexamined Patent Publication JP-A 11-183613, JP-A 2000-171556 and JP-A 2001-74829.
Examples of the radar apparatus of the related art and the radar-apparatus mounted vehicle having the radar apparatus mounted thereon are disclosed in Japanese Unexamined Patent Publication JP-A 2003-35768, for example.
In any of the configurations disclosed in Japanese Unexamined Patent Publications JP-A 2000-258525, JP-A 11-183613, JP-A 2000-171556 and JP-A 2001-74829, however, a portion of the local signal L0 is reflected on the mixer 66 and then leaks from the third terminal 64c of the circulator 64 to the first terminal 64a, as shown in FIG. 34. Moreover, this high-frequency signal having leaked is totally reflected on the pulse modulator 63 in the OFF state so that it is transmitted as the unnecessary high-frequency signal from the transmitting/receiving antenna 65. As a result, the ON/OFF ratio or the ratio of the intensities of the individual transmission high-frequency signals, which are transmitted from the transmitting/receiving antenna 65 when the pulse modulator 63 are ON and OFF, drops to raise a problem that the transmission/reception performance drops. Specifically, this unnecessary high-frequency signal migrates, if transmitted, into the high-frequency signal RF to be intrinsically received, thereby to raise a problem that the portion of the high-frequency signal RF to be received cannot be correctly received.
In the radar apparatus using that high-frequency transmitter-receiver, moreover, the high-frequency signal of a weak intensity reflected on a distant objective to be detected is buried in the high-frequency signal or the noise transmitted when the pulse modulator 63 is OFF. As a result, the range to be detected may be narrowed, or an erroneous detection may occur thereby to cause a problem that the detection of the objective is delayed.
Moreover, the vehicle or the small-sized boat having such radar apparatus mounted thereon is caused to take a proper behavior such as an avoidance or a braking by detecting the objective with the radar apparatus on the basis of the detected information. However, the detection of the objective is delayed to raise a problem that the delayed detection may cause an abrupt behavior in the vehicle or the small-sized boat.